A fuel cell is an electric power generation system for producing electrical energy through a chemical reaction between oxidant and hydrogen or a hydrocarbon-based material such as methanol, ethanol, or natural gas.
A fuel cell can be classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type depending upon the kind of electrolyte used. Although each of these different type of fuel cells operates in accordance with the same basic principles, they may differ from one another in the kind of fuel, the operating temperature, the catalyst, and the electrolyte used.
Recently, a polymer electrolyte membrane fuel cell (PEMFC) has been developed. The PEMFC has power characteristics that are superior to those of conventional fuel cells, as well as a lower operating temperature and faster start and response characteristics. Because of this, the PEMFC can be applied to a wide range of applications such as for portable electrical power sources for automobiles, distributed power sources for houses and public buildings, and small electrical power sources for electronic devices.
A PEMFC is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a body of the PEMFC, and the fuel pump provides the fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate the hydrogen gas and supplies the hydrogen gas to the stack. Fuel stored in the fuel tank is pumped to the reformer using power which can be provided by the PEMFC. Then, the reformer reforms the fuel to generate the hydrogen gas which is electrochemically reacted with oxidant in the stack to generate the electrical energy.
Alternatively, a fuel cell may include a direct oxidation fuel cell (DOFC) in which a liquid methanol fuel is directly introduced to the stack. Unlike a PEMFC, a DOFC does not require a reformer.
In the above-mentioned fuel cell system, the stack for generating the electricity has a structure in which several unit cells, each having a membrane-electrode assembly (MEA) and a separator (referred to also as “bipolar plate”), are stacked adjacent to one another. The MEA is composed of an anode (referred to also as “fuel electrode” or “oxidation electrode”) and a cathode (referred to also as “air electrode” or “reduction electrode”) separated by a polymer electrolyte membrane.
FIG. 1 is a diagram showing an operation of a fuel cell 1. The fuel cell 1 includes an anode 3, a cathode 5, and a polymer electrolyte membrane 7. As shown, when hydrogen gas or fuel is supplied to the anode 3, an electrochemical oxidation reaction occurs and hydrogen is ionized to thereby produce protons H+ and electrons e−. The protons H+ move to the cathode 5 through the polymer electrolyte membrane 7 and the electrons e− move to the cathode 5 through an external circuit (not shown). The protons H+ in the cathode 5 electrochemically react with an oxidant such as oxygen or air supplied to the cathode 5 to thereby produce reaction heat and water, and the movement of the electrons e− through the external circuit produces electrical energy. The electrochemical reactions in the fuel cell 1 can be expressed as follows.Anode reaction: H2→2H++2e−Cathode reaction: 2H++1/2O2+2e−→H2O
A polymer membrane-electrode assembly is composed of a solid polymer electrolyte membrane (e.g., the membrane 7 of FIG. 1) and an electrode layer including catalysts supported on carbon. The polymer electrolyte membrane can be fabricated using a perfluorosulfonic acid ionomer membrane such as Nafion® (by DuPont), Flemion® (by Asahi Glass), Asiplex® (by Asahi Chemical), or Dow XUS® (by Dow Chemical). The electrode layer including the catalysts supported on the carbon can be fabricated by binding the electrode substrates such as a porous carbon paper or a carbon cloth with a carbon powder carrying a pulverized catalyst particle such as platinum (Pt) or ruthenium (Ru) using a waterproof binder.
Conventional polymer electrolyte membranes (referred to also as “polymer membranes”) such as Nafion® have good proton conductivity (or permeability), good chemical-resistance, and good anti-corrosiveness. However, they are expensive, and may allow methanol to crossover. Also, since the movement of H+ or protons through the conventional polymer membranes requires water, a humidifier needs to be used. Therefore, a setup cost for a fuel cell using a conventional polymer membrane is high and a large setup space is also required. In addition, when the fuel cell is operated at a high temperature, the moisture is evaporated and thus the proton conductivity of the conventional polymer membrane is degraded.